I always try to take good notes of my darkroom sessions. I try to record everything, and have a digital timer so my exposures are accurate. I try to be consistent always using with minor exceptions Ilford MG IV fiber paper and Dektol developer. I find when I go back to a negative using the same exposure,f-stop, burning etc there are more than minor differences almost to the point I am starting all over again. The exposures differ at times typically to 25% +- and also notice differences in contrast. Am I doing something wrong?